1. Field of the Invention
The present invention relates to an optical fiber preform suspending and supporting apparatus for suspending and supporting a porous optical fiber preform at its upper end, and an optical fiber preform processing apparatus using the optical fiber suspending and supporting apparatus, in a process of producing a porous optical fiber preform on a starting preform by a VAD method or an OVD method or in a process after the above one for vitrifying (dehydrating and sintering) the porous optical fiber preform.
2. Description of the Related Art
An optical fiber comprised of a core and a cladding is formed by heating and drawing a vitrified optical fiber preform.
The method for producing such an optical fiber preform will be described below. First, a core portion porous optical fiber preform is deposited and formed at the lower end of a starting preform (a seed rod) by the VAD method. Then, this core portion porous optical fiber preform is heat treated in a vitrification (dehydration and sintering) step to obtain a transparent core use optical fiber preform. Next, using this transparent core use optical fiber preform as a starting preform, a cladding portion porous optical fiber preform is deposited and formed by the OVD method. Next, the cladding portion porous optical fiber preform is heated in a vitrification (dehydration and sintering) step to obtain a transparent optical fiber preform.
The resultant transparent vitrified optical fiber preform is heated and drawn to form an optical fiber comprised of for example a core of a diameter of 10 xcexcm and a cladding of a diameter of 125 xcexcm at the circumference of the core.
When producing such a core portion porous optical fiber preform and cladding portion porous optical fiber preform, the porous optical fiber preforms are suspended and supported at the upper end by an optical fiber preform suspending and supporting apparatus. As known optical fiber preform suspending and supporting apparatuses, there are a pin supporting type, a chuck supporting type, and a combined type of the two.
FIG. 1 is a vertical sectional view of the structure of a pin supporting type optical fiber preform suspending and supporting apparatus 5A. Specifically, the optical fiber preform suspending and supporting apparatus 5A shown in FIG. 1 is configured to be housed in a dehydration and sintering furnace, as an example of an optical fiber preform processing apparatus, used when vitrifying a porous glass layer 8 for cladding to make it transparent in a vitrification (dehydration and sintering) step of a porous optical fiber preform 6 comprising a core use optical fiber preform, that is, a starting preform 7, and a porous glass layer 8 for cladding produced on the starting preform 7 by the OVD method.
In the dehydration and sintering furnace schematically illustrated in FIG. 1, a pin supporting type optical fiber preform suspending and supporting apparatus 5A is provided so that a main shaft 4 passes through a hole 3 in the upper lid 2 of a furnace tube 1 made of silica-based glass or the like. A not illustrated rotation and support mechanism is provided at the upper portion of the main shaft 4. The rotation and support mechanism supports the main shaft 4 from which the porous optical fiber preform 6 is suspended and rotates the porous optical fiber preform 6 by rotating the main shaft 4.
In this way, the porous optical fiber preform 6 is supported and suspended by the main shaft 4 at the upper end of the starting preform 7 by the optical fiber preform suspending and supporting apparatus 5A.
In this case, the starting preform 7 is the transparent core use optical fiber preform formed on a seed rod. A porous glass layer 8 for cladding is deposited at the circumference of the starting preform 7.
The optical fiber preform suspending and supporting apparatus 5A is structured so that the upper end of the starting preform 7 is inserted inside a supporting cylinder 9 at the lower end of the main shaft 4 and a pin 12 is inserted into through holes 10 and 11 formed horizontally in the supporting cylinder 9 and starting preform 7 at the portion where they are fit together so that the starting preform 7 can swing around the pin 12.
At the circumference of the furnace tube 1, a heater 13 for heating and vitrifying the porous optical fiber preform 6 is provided.
Such an optical fiber preform suspending and supporting apparatus 5A is housed at the inside of the high temperature furnace tube 1 to vitrify the porous optical fiber preform, so it is made of heat resistant silica-based glass or the like so as to prevent the invasion of impurities into the porous optical fiber preform 6.
In the pin supporting type optical fiber preform suspending and supporting apparatus 5A shown in FIG. 1, since the through holes 10 and 11 are formed in the supporting cylinder 9 and the upper portion of the starting preform 7 and the pin 12 is passed through the through holes 10 and 11 to support the porous optical fiber preform 6 by the main shaft 5, it suffers from the disadvantage of a degraded mechanical strength of the starting preform 7 in which the through hole 11 is formed. In particular, recently large-sized porous optical fiber preforms 6 have been formed. Thus, the porous optical fiber preforms 6 have become heavier too, so the decline of the mechanical strength of the starting preform 7 becomes a problem.
Furthermore, due to the restriction of the mechanical strength of the starting preform 7, the diameter of the through hole 11 formed in the porous optical fiber preform 6 through which the pin 12 is passed cannot be made too large. Therefore it suffers from another disadvantage that since the diameter of the pin 12 is therefore also small, when placed in the high temperature furnace tube 1 for vitrification, the pin 12 is liable to deform due to the heat from the heater 13.
In order to overcome these disadvantages, for example, Japanese Patent No. 2683757 proposed to provide a gas passage along the axial line of the main shaft 4 and supply a cooling gas through the gas passage to cool the pin 12 and therefore prevent a decline of the mechanical strength of the pin supporting type optical fiber preform suspending and supporting apparatus 5A. In such a configuration, however, it is needed to provide means for controlling the flow rate of the cooling gas, preventing reverse flow by controlling the internal pressure of the reaction vessel in the furnace tube, and preventing leakage at the rotating connecting portion of a gas joint of the tube for conducting the cooling gas. Consequently, it suffers from another disadvantage that the operation and maintenance of the dehydration and sintering furnace become rather expensive.
FIG. 2 is a vertical sectional view of the structure of a chuck supporting type optical fiber preform suspending and supporting apparatus 5B. In the same way as FIG. 1, FIG. 2 shows an example of the case where an optical fiber preform suspending and supporting apparatus 5B is housed in a dehydration and sintering furnace for vitrifying a cladding portion porous optical fiber preform produced by the OVD method in a vitrification (dehydration and sintering) step.
In the chuck supporting type optical fiber preform suspending and supporting apparatus 5B, a holding portion 14 comprising a chuck is integrally provided at the lower end of the main shaft 4. The holding portion 14 is provided with an insertion space 15 open at one side and with a slit 16 for insertion of a starting preform at a supporting portion 14a at the bottom of the insertion space 15.
Such an optical fiber preform suspending and supporting apparatus 5B, as described previously with reference to FIG. 1, is also made of silica-based glass to prevent the invasion of impurities into the porous optical fiber preform 6.
When using the chuck supporting type optical fiber preform suspending and supporting apparatus 5B illustrated in FIG. 2, the porous optical fiber preform 6 comprised of the core use optical fiber preform (starting preform) 7 and the porous glass layer 8 deposited on it is formed with an enlarged-diameter portion 7a at the upper end of the starting preform 7. The enlarged-diameter portion 7a of the starting preform 7 is inserted into the starting preform insertion slit 16. By supporting the enlarged-diameter portion 7a at the upper end of the starting preform 7 by the supporting portion 14a of the chuck holding portion 14, the porous optical fiber preform 6 is supported and suspended downward.
In the chuck supporting type optical fiber preform suspending and supporting apparatus 5B illustrated in FIG. 2, however, it is difficult to precisely machine a shape like the enlarged-diameter portion 7a at the upper end of the starting preform 7. Generally the machining accuracy of the enlarged-diameter portion 7a is about xc2x10.2 mm, so it suffers from the disadvantage that with such a machining accuracy, it is difficult to precisely and reproducibly suspend the starting preform 7 vertically by the chuck 14.
Consequently, as illustrated in FIG. 3A, the porous optical fiber preform 6 swings, along with rotation of the main shaft 4, inclined relative to the vertical line of the main shaft 4 at an angle xcex8 of, for example, about xc2x10.2 degree. Therefore, the vitrified (dehydrated and sintered) optical fiber preform 6 bends as shown in FIG. 3B. An optical fiber produced from such an optical fiber preform 6 ends up with a core offset from the center line or a non-circular cladding causing degradation of the quality of the finally produced optical fiber.
FIG. 4A and FIG. 4B are vertical sectional views of the structure of an optical fiber preform suspending and supporting apparatus 5C of a type combining the pin supporting type and chuck supporting type viewed from two locations 90 degrees different from each other with respect to the axial center of the main shaft 4.
In the optical fiber preform suspending and supporting apparatus 5C illustrated in FIG. 4A and FIG. 4B, the lower end of the main shaft 4 is fitted into a cylindrical movable connector 17. Through holes 18 and 19 are formed in the two of them in the horizontal direction. A pin 12 is inserted into the through holes 18 and 19 to join them so that the movable connector 17 is supported by the main shaft 4 and is swingable around the pin 12. A holding portion 14 comprising a chuck is integrally formed at the bottom part of the movable connector 17 to hold the inclined portion 7b and enlarged-diameter portion 7a at the upper end of the starting preform 7. The inside of the holding portion 14 is shaped to accept the inclined portion 7b and enlarged-diameter portion 7a of the starting preform 7. An insertion space 15 open at one side is formed in the holding portion 14. A slit 16 for insertion of a starting preform is provided in a supporting portion 14a at the bottom of the insertion space 15 of the holding portion 14.
A number of carbon screws are screwed into the movable connector 17 and the holding portion 14 to adjust the orientation of the main shaft 4 and the center axis of the starting preform 7 to coincide.
In the optical fiber preform suspending and supporting apparatus 5C combining the pin supporting type and chuck supporting type illustrated in FIG. 4A and FIG. 4B, since the holding portion 14 is connected swingably with respect to the main shaft 4 by the pin 12 through the movable connector 17, it is not necessary to highly precisely machine the shapes of the enlarged-diameter portion 7a and the inclined portion 7b at the upper end of the starting preform 7. Even with a machining accuracy of about xc2x10.2 mm, the porous optical fiber preform 6 can be supported suspended without being inclined relative to the vertical line of the main shaft 4 utilizing its own weight.
However, rather complicated machining is required to shape the upper portion of the starting preform 7 into the inclined portion 7b and the enlarged-diameter portion 7a of the starting preform 7. Further, rather complicated machining is also required to shape the inside of the holding portion 14 into a shape mating with the inclined portion 7b and the enlarged-diameter portion 7a. 
Further, since the pin 12 is made a thin one to pass through the through holes 18 and 19 formed in the main shaft 4 and the movable connector 17, it suffers from the disadvantage that it will easily deform when placed in a high temperature environment such as the furnace tube 1.
Because a number of carbon screws are used to adjust the orientation of the main shaft 4 and the position of the axial center of the starting preform 7 to coincide, not only do the movable connector 17 and the holding portion 14 become complicated in structure, but also positioning work is required.
An object of the present invention is to provide an optical fiber preform suspending and supporting apparatus able to prevent deformation of a pin placed in a high temperature environment and able to support a porous optical fiber preform without having an adverse influence on supports of the pin and without resulting in inclination relative to the vertical line of a rotating main shaft.
Another object of the present invention is to provide an optical fiber processing apparatus employing such an optical fiber preform suspending and supporting apparatus.
According to a first aspect of the present invention, there is provided an optical fiber preform suspending and supporting apparatus comprising a cylindrical movable connector for housing a columnar lower end of a main shaft, circular through holes formed in order to pass horizontally through a lower end of a main shaft and a movable connector, a columnar pin which passes through the through holes to connect the movable connector with the main shaft and supports the movable connector swingably with respect the main shaft about the pin, and a holding portion having an insertion space integrally formed with the movable connector, positioned at the bottom portion of the movable connector, and housing a columnar enlarged-diameter portion of the upper end of a support of an optical fiber preform or a porous optical fiber preform and a supporting portion which defines the bottom surface of the insertion space, is formed an insertion slit for passing the support of an optical fiber preform or a porous optical fiber preform, and supports a flat bottom of the columnar enlarged-diameter portion of the support of an optical fiber preform or a porous optical fiber preform. In the above apparatus, the center lines of the main shaft, the movable connector, the holding portion, and the supporting portion coincide. The difference between the outside diameter of the columnar pin and the outside diameter of the lower end of the main shaft, the difference between the outside diameter of the lower end of the main shaft and the inside diameter of the movable connector, and the difference between the outside diameter of the enlarged-diameter portion of the support of an optical fiber preform or a porous optical fiber preform and the inside diameter of the holding portion are defined so that the movable connector is swingable around the axis of the pin.
Preferably, the outside diameter of the pin ranges from 25% to 50% of the outside diameter of the lower end of the main shaft. More preferably, the outside diameter of the pin ranges from 30% to 40% of the outside diameter of the lower end of the main shaft.
Preferably, the clearance between the through holes and the pin is not more than 1 mm and not less than 0.02 mm. More preferably, the clearance between the through holes and the pin is not more than 0.5 mm and not less than 0.02 mm.
Preferably, the clearance between the lower end of the main shaft and the movable connector is in the range of 0.3 mm to 1.0 mm.
Preferably, the clearance between the holding portion and the enlarged-diameter portion of the support is in the range of 0.3 mm to 1.0 mm.
According to a second aspect of the present invention, there is provided an optical fiber preform suspending and supporting apparatus comprising a cylindrical movable connector for housing a columnar lower end of a main shaft, first circular through holes formed so as to horizontally pass through a lower end of the main shaft and the movable connector, a first columnar pin which passes through the first through holes to connect the movable connector with the main shaft and supports the movable connector to the main shaft swingably around the first pin, a holding portion including a columnar upper portion inserted into the lower portion of the cylindrical movable connector, an insertion space for housing a columnar enlarged-diameter portion of the upper end of a support of an optical fiber preform or a porous optical fiber preform, and a supporting portion which defines the bottom surface of the insertion space, is formed with an insertion slit for passing the support of an optical fiber preform or a porous optical fiber preform, and supports a flat bottom of the columnar enlarged-diameter portion of the support of an optical fiber preform or a porous optical fiber preform, second circular through holes positioned below the first through holes formed in the movable connector, perpendicularly intersecting the orientation of the first through holes, and horizontally passing the columnar upper portion of the holding portion and the movable connector, and a second columnar pin which passes through the second through holes to connect the columnar upper portion of the holding portion with the movable connector and supports the holding portion to the movable connector swingably around the second pin. In this apparatus, the center lines of the main shaft, the movable connector, the holding portion, and the supporting portion coincide. The difference between the outside diameter of the first columnar pin and the outside diameter of the lower end of the main shaft, the difference between the outside diameter of the lower end of the main shaft and the inside diameter of the movable connector, the difference between the outside diameter of the second columnar pin and the outside diameter of the columnar upper portion of the holding portion, and the outside diameter of the enlarged-diameter portion of the support of an optical fiber preform or a porous optical fiber preform and the inside diameter of the supporting portion of the holding portion are defined so that the movable connector is able to swing around the axis of the first pin.
Preferably, the outside diameter of the first pin ranges from 25% to 50% of the outside diameter of the lower end of the main shaft. More preferably, the outside diameter of the first pin ranges from 30% to 40% of the outside diameter of the lower end of the main shaft.
In addition, preferably, the outside diameter of the second pin ranges from 25% to 50% of the outside diameter of the upper portion of the holding portion. More preferably, the outside diameter of the second pin ranges from 30% to 40% of the outside diameter of the upper portion of the holding portion.
Preferably, the clearance between the first through holes and the first pin is not more than 1 mm and not less than 0.02 mm. More preferably, the clearance between the first through holes and the first pin is not more than 0.5 mm and not less than 0.02 mm.
In addition, preferably, the clearance between the second through hole and the second pin is not more than 1 mm and not less than 0.02 mm. More preferably, the clearance between the second through hole and the second pin is not more than 0.5 mm and not less than 0.02 mm.
Preferably, the clearance between the lower end of the main shaft and the movable connector is in the range of 0.3 mm to 1.0 mm, the clearance between the columnar upper portion of the holding portion and the movable connector is in the range of 0.3 mm to 1.0 mm, and the clearance between the holding portion and the enlarged-diameter portion of the support is in the range of 0.3 mm to 1.0 mm.
Preferably, the inside diameter of the first through holes is equal to that of the second through holes and the outside diameter of the first pin is equal to that of the second pin.
According to a third aspect of the present invention, there is provided an optical fiber preform processing apparatus employing an optical fiber preform suspending and supporting apparatus of the first aspect of the present invention.
According to a fourth aspect of the present invention, there is provided an optical fiber preform processing apparatus employing an optical fiber preform suspending and supporting apparatus of the second aspect of the present invention.